The streets blocked off as officials investigate the scene where a pedestrian was struck and killed by a MUNI bus at the corner of 18th street and Hartford in the Castro neighborhood on Friday August 19, 2011, in San Francisco, Ca.

Photo: Michael Macor, The Chronicle

The streets blocked off as officials investigate the scene where a...

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The streets blocked off as officials investigate the scene where a pedestrian was struck and killed by a MUNI bus at the corner of 18th street and Hartford in the Castro neighborhood on Friday August 19, 2011, in San Francisco, Ca.

Photo: Michael Macor, The Chronicle

The streets blocked off as officials investigate the scene where a...

Image 3 of 5

The woman's body is removed from the scene after she was struck and killed by a MUNI bus at the corner of 18th street and Hartford in the Castro neighborhood on Friday August 19, 2011, in San Francisco.

Photo: Michael Macor, The Chronicle

The woman's body is removed from the scene after she was struck and...

Image 4 of 5

Head of MUNI, Ed Reiskin on the scene where a pedestrian was struck and killed by a MUNI bus at the corner of 18th street and Hartford in the Castro neighborhood on Friday August 19, 2011, in San Francisco.

Photo: Michael Macor, The Chronicle

Head of MUNI, Ed Reiskin on the scene where a pedestrian was...

Image 5 of 5

People watch at the scene where a pedestrian was struck and killed by a MUNI bus at the corner of 18th street and Hartford in the Castro neighborhood on Friday August 19, 2011, in San Francisco, Ca.

Sgt. Michael Andraychak said the bus, which was acting as an F-Line shuttle, made a left turn onto Hartford en route to pick up passengers at Castro and Market streets. He said witnesses reported that the woman had nearly reached the opposite curb when the bus struck her.

"The driver is shaken up by this," Andraychak said.

Nearby residents who gathered at the intersection said it was uncommon to see a bus turn onto Hartford.

But police said when too many passengers crowd the F-Line trolley cars that run down Market Street, a street vehicle will sometimes get pulled from its normal route and sent there to ease passenger congestion. That was the case Friday afternoon, Andraychak said, adding that no passengers were aboard when the accident occurred.

"This is unusual because there are no buses on Hartford," said Paul Benson, a bartender at nearby Moby Dick who said he saw the driver stop in the intersection, get out, and look around as if he'd hit something. "I thought, 'What is he doing?' "

The incident drew curiosity and sympathy from onlookers, as some crossed themselves and others reflected that it could have been them.

The driver is undergoing routine drug and alcohol tests, police said.

"At this point there is no reason to believe he was impaired," Andraychak said.